1. Field of the Invention
The present invention relates to an optical film imparted property of preventing glare, which is used in a polarizing film of an image displaying device, and a process for preparing the same.
2. Description of the Related Art
Visibility of an image displaying device including a liquid crystal displaying device is remarkably deteriorated when the outdoor daylight is reflected on its image displaying surface. In utilities requiring high quality of image such as a television and a personal computer, utilities used in outdoor where the outdoor light is intense such as a video camera and a digital camera, and utilities which perform display utilizing the reflected light such as a reflecting-type liquid crystal displaying device such as a portable phone, the surface of a displaying device is generally treated for preventing the reflecting thereon. Reflecting on preventing treatment is roughly classified into non-reflecting treatment by interference between layers of an optical multilayered film, and so-called glare preventing treatment by forming fine irregularities on the surface to scatter the incident light, and gradating a reflected on image. The cost of former non-reflecting treatment is high due to necessity of formation of a multilayered film having a uniform optical film thickness. To the contrary, since the latter glare preventing treatment can be realized at a relatively low cost, the treatment is used for, for example, a large scale personal computer and a large scale monitor. Glare preventing treatment is also called non-glare treatment or anti-glare treatment, and has previously been performed, for example, by a method of coating, on a transparent substrate, a ultraviolet curable resin with a filler dispersed therein, drying this, irradiating with ultraviolet-ray to cure the resin, whereby, the filler shape is transferred to the surface of the film to form random irregularities. Thus obtained layer provided with property of preventing glare (anti-glare property) is also called non-glare layer or anti-glare layer.
Previously, there have been many proposals for forming fine irregularities on the surface of the film used in an image displaying device to impart property of preventing glare. For example, JP-A No. 7-181306 and JP-A No. 5-341123 describe a non-glare layer composed of a cured film of a ultraviolet curable resin containing a fine particle as a filler.
In addition, JP-A No. 6-16851 describes a method of obtaining a scratch-resistance anti-glare film with an anti-glare layer formed thereon, by using a mat-like shaping film having fine irregularities on the surface, coating an ionizing radiation curable resin composition or a thermosetting resin composition on a transparent substrate, laminating the aforementioned shaped film on this coating film in the uncured state, irradiating with ionizing radiation or heating to cure a coating film, and peeling the shaping film.
JP-A No. 2003-4903 discloses a glare preventing film having an anti-glare layer on a transparent support and having a concave part and a convex part on the surface, wherein an area of a cross-sectional side of each concave part is 1,000 μm2 or less. Herein, a ultraviolet curable resin in which a particle having an average particle diameter of 0.2 to 10 μm is dispersed is coated on a transparent support, and this is cured with ultraviolet ray to prepare an anti-glare film having irregularities.
However, when an anti-glare layer is constructed of a cured ultraviolet curable resin containing a filler, to manifest sufficient anti-glare property, since back scattering of the outdoor daylight due to a filler occurs, and the reflected light is distributed in a wide range, a wide range of a scattered component of the outdoor daylight is observed in a direction of an observer, and so-called whitening is observed in which a whole of a screen becomes white.
In addition, in an even a method of obtaining anti-glare surface having no filler by a method of transferring a shape using a mat-like shaping film, since a reflected light distribution depends on a shape of a shaping film as a template, it can not be said that this method is not necessarily sufficient to prevent whitening. JP-A No. 6-16851 describes that a matting agent such as a filler is not used in an anti-glare layer. However, as a method of forming irregularities on a shaping film, only a method of coating a composition composed of a filler and a binder resin on a substrate film to transfer the shape of the filler is disclosed and, since distribution of a filler and, consequently, arrangement of irregularities becomes random, it is thought that sufficient prevention of whitening can not be performed.
Generally, as a method of reducing whitening, formulation of reducing an amount of a filler to be added in order to reduce a diffused component of the reflected light, and formulation of downsizing an average particle diameter of a filler are contemplated. However, according to these formulations, a proportion of a flat surface of an anti-glare surface is increased at the same time, causing clarity of an image of the reflected on outdoor daylight, and anti-glare property is deteriorated.
On the other hand, JP-A No. 2002-365410 discloses an optical film having the surface on which fine irregularities are formed, wherein when the light is introduced on the surface of the film from a direction at −10° relative to a normal, and only the light reflected from the surface is observed, a profile of the reflected light satisfies a specified relationship. And, it is described that, in order to give such the reflection profile, it is preferable that the surface is of a multiple spherical shape having substantially no flat part.
In addition, although not directly related to an anti-glare optical film, the technique of etching the surface of a glass with hydrofluoric acid is known. For example, JP-A No. 2002-90732 describes that a concave part having an average diameter of around 6 μm is formed on the surface of a glass by two-stage etching using hydrofluoric acid, and a glass on which irregularities are formed like this is used as a reflection plate for a liquid crystal display panel. A most general glass is a soda-lime glass. However, the previously known glare preventing-treated film has not necessarily sufficient effect of preventing the aforementioned whitening, and it can not be said that sufficient visibility is obtained.